The present invention relates to a light emitting device which employs a semiconductor laser as a light source, and more particularly to improvement of its noise characteristic.
Semiconductor lasers have now found use in a variety of applications such as an optical disc and an optical printer, not to speak of optical communications. Especially, they are playing an important part as a light source for optical fiber communication because their optical output can be directly modulated, with ease, at a rate of several hundred megabits per second or more by changing the values of their injection current. Since the semiconductor lasers are two-terminal elements, however, they are susceptible to the influence of reflected light; even 1% reflected light will produce large noise in their optical output. As the coherence of the output light improves, the influence of the reflected light becomes more marked, posing a serious problem in a single wavelength laser such as a distributed feedback (DFB) laser. Furthermore, in case of obtaining an oscillation line-width of a narrow modulation band by modulating the output of the single wavelength laser with an external optical modulating element, it is indispensable to take measures against noises which are caused by the reflected light. Accordingly, it is no exaggeration to say that noise-control measures in the light emitting device are of primary importance to the success of future optical transmission systems which employ an external optical modulating element, such as a high-speed optical transmission system of a transmission rate in excess of gigabits per second and a coherent optical transmission system.
A conventional remedy for noise attributable to reflected light is merely to minimize the reflection of light or alleviate the reflected light by means of an optical isolator. With this method, however, the introduction of the optical isolator impairs not only the overall reliability of the light emitting device but also the efficiency of optical coupling to an optical fiber or the like. Further, the reduction of the reflected light is also limited, and at the present technical level, reflected light of the order of 0.1 to 1% is unavoidable. Therefore, there is a strong demand for a light emitting device which is insusceptible to the influence of the reflected light.